1. Field of the Invention
This invention relates to a new aliphatic ester diisocyanate, to a process for its production and to its use in the production of plastics by the isocyanate polyaddition process.
2. Description of the Prior Art
Diisocyanates containing ester groups are known. Compounds of this type are described, for example, in DE-OS 1,618,798. The ester diisocyanates are produced by esterification of aminocarboxylic acids with certain alkanolamines and reaction of the resulting ester diamines or their hydrochlorides with phosgene. The aminocarboxylic acid used in this process may be, for example, 6-aminocaproic acid which is inexpensively obtainable from .epsilon.-caprolactam. However, DE-OS 1,618,798 itself shows that the use of the alkanolamines mentioned therein is accompanied by certain disadvantages. Additional difficulties were encountered in the reproduction of Examples 7 and 8 of DE-OS 1,618,798 (see following Comparison Examples 1 and 2). Thus, the dihydrochlorides prepared from the alkanolamines mentioned as preferred accumulate in the form of pasty, solid-liquid mixtures which are insoluble in the usual organic solvents. Isolation, for example by careful removal of the solvent, leads to further decomposition of the products. It follows from this that purity testing of the dihydrochlorides is not possible and that, in particular, any purification necessary cannot be carried out. A modified procedure for the production of the dihydrochlorides using 6-aminocaproyl chloride hydrochloride as starting material, for example according to H. Ulrich et al, Liebigs Ann. der Chemie, 1975, page 1317, does not produce any improvement. In this process also, the reaction products with the alkanolamines mentioned as preferred in DE-OS 1,618,798 are obtained as greasy products which cannot be worked up or purified (see following Comparison Example 3) and, therefore, cannot be reacted to give pure diisocyanates. Due to the thermal instability of the dihydrochlorides, the exchange for the solvent required for phosgenation is also very difficult.
The phosgenation in o-dichlorobenzene described in Example 8 of DE-OS 1,618,798 is not carried out in the usual way under reflux conditions, so that isocyanates having high contents of bound chlorine are obtained, as also mentioned in this Example.
However, when phosgenation is carried out under reflux conditions, even the use of a low-boiling solvent (see Comparison Example 3 below) fails to give useful yields or pure products because the thermal instability makes purification impossible. If, by contrast, phosgenation is carried out under reflux in o-dichloro-benzene, it only leads to decomposition products and not to significant yields of isocyanate.
These disadvantages would appear to be the reason why the diisocyanates specifically described in DE-OS 1,618,798 have not yet acquired any commercial significance.
An object of the present invention is to provide a new diisocyanate containing ester groups wherein its production is not attended by any of the above-described disadvantages, i.e., which is readily obtainable in high yields from inexpensive starting materials and which by virture of its properties is eminently suitable as an isocyanate component in the production of polyisocyanate polyaddition products, preferably polyurethane plastics. This object may be achieved in accordance with the present invention.